Duplicate library and language feature gates silently fail

[dlrobertson]

Summary

If a language feature and a library feature are given the same name ![feature(<name>)] will only evaluate the language feature.

Expected Result

There should either be some sort of error that is thrown if both a language feature and a library feature have the same name, or both should be evaluated.

[Centril]

Seems like something that could go into tidy? cc @rust-lang/infra

[varkor]

It should be straightforward to make this an error. I'll look into it.

On second thoughts, we probably want to allow lang and lib features to share names.